Conventional vehicular air-conditioning systems typically use a vapor-compression refrigeration system, which includes a closed circuit (refrigeration cycle) having a compressor, a condenser, an expansion valve and an evaporator. Refrigerant circulates through the closed circuit. The refrigerant vaporizes as it absorbs heat from air within a passenger compartment of a vehicle to thereby cool the air.
More specifically, the refrigerant in a liquid state passes through an expansion valve where the refrigerant undergoes adiabatic expansion and decreases in pressure and temperature. Thereafter, the refrigerant in the form of mist enters an evaporator and vaporizes upon absorbing heat from air within a passenger compartment. The heated vapor refrigerant is then fed into a compressor where the vapor refrigerant undergoes adiabatic compression and turns into a high-temperature/high-pressure gaseous state. The gaseous refrigerant then advances to a condenser to release heat outwardly and condense into a liquid state, which liquid refrigerant is returned to the expansion valve.
Such a refrigeration cycle is also used in an air-conditioning system for a vehicle carrying an idling-stop system. The idling-stop system is designed to temporarily stop idling of an engine when a vehicle state satisfies predetermined stop conditions to thereby reduce fuel consumption. In such a vehicle, the engine cannot drive the compressor during its idling-stopped state. Thus, the compressor needs to be driven by an electric motor rather than by the engine. An electric motor designed for such a purpose is known from, for example, JP-A-6-323649. Speed of the known electric motor is controlled in correspondence with a temperature at an outlet of the evaporator.
When outside air is introduced into the air-conditioning system during the idling-stopped state of the engine, the introduced air may be influenced by heat from an engine compartment side. As a result, when the air passes through an outlet of the evaporator, the air may exhibit a temperature about 10° C. greater than when it began being introduced into the air-conditioning system. In this instance, the electric motor whose speed is controlled in correspondence with a temperature of the air at the outlet of the evaporator can be driven under an unnecessary condition or at an unnecessarily high speed due to the greater air temperature at the outlet of the evaporator, thus resulting in excessive consumption of electric power.